1. Technical Field
The present invention generally relates to a nonvolatile memory apparatus, and more particularly, to a nonvolatile memory apparatus and a method for driving the same.
2. Related Art
In general, a semiconductor memory apparatus is classified into volatile memory apparatuses and nonvolatile memory apparatuses, depending on whether the memory apparatus can maintain data stored therein when power is cut off. The volatile memory apparatuses include random access memories such as SRAM and DRAM, and the nonvolatile memory apparatuses include read only memories and flash memories such as EEPROM.
Recently, a variety of memory apparatuses capable of storing data using a resistor have been proposed as an example of the nonvolatile memory apparatus. For example, a phase change RAM, a resistive RAM, a magnetic RAM and the like belong to the nonvolatile memory apparatuses. While a DRAM or flash memory stores data using electric charges, the nonvolatile memory apparatus using a resistor stores data using on phase change of a phase change material, resistance change of a variable resistor, and resistance change of a magnetic tunnel junction (MTJ) layer depending on the magnetization state of a ferromagnetic material.
Specifically, the phase change RAM stores data using a phase change material such as germanium-antimony-tellurium (GST; Ge2Sb2Te5), which changes into a crystalline state or amorphous state while cooled down after being heated. Since a phase change material in a crystalline state has low resistance and a phase change material in an amorphous state has high resistance, the crystalline state may be defined as a set state or logic level 0, and the amorphous state may be defined as a reset state or logic level 1.
The phase change RAM provides a set pulse or reset pulse to a phase change material, and writes data using Joule heat generated by the set pulse or reset pulse. Specifically, the phase change material may be heated to a melting temperature or more using the reset pulse, and then rapidly quenched to change into the amorphous state. Alternatively, the phase change material may be heated to a temperature ranging from a crystallization temperature to a melting temperature using the set pulse, and then quenched to be changed into the crystalline state after the temperature is maintained during a predetermined time.
In order to prevent unnecessary current consumption, the nonvolatile memory apparatus using a resistor, including the phase change RAM, may adopt a write verify operation during a write operation. That is, when input data is to be written, data stored in a corresponding variable resistance memory cell (for example, a phase change memory cell in the case of the phase change RAM) is first read. Then, the read data and the input data to be written are compared to each other. Only when the two data are different from each other, the data is written into the corresponding memory cell. Such an operation is repeated until a desired level of data is stored in the memory cell.
However, when the write operation including a verification read operation is performed, the operation of reading stored data and the operation of writing data are separately performed. Therefore, the entire write operation time of the semiconductor memory apparatus inevitably increases.